1. Technical Field
The invention relates generally to semiconductor devices and more specifically to semiconductor receiver devices.
2. Related Art
As CMOS semiconductor devices becomes increasingly complex, two or more voltage supplies are often needed on each chip to provide for devices operating at different operating levels. For example, an off-chip receiver usually operates at a different operating value than the internal circuitry of the chip. Current hardware will allow for multiple voltages on the same chip.
Most circuits have been designed to allow for correct operation while both voltages are on, or powered-up in a certain sequence. However, during some portion of the operational period of the system, the voltage for a particular device may be turned off. With prior devices that had a single power supply, turning off a particular device resulted in no damage to the device, but with multiple power devices, a risk may exist if the power supplies are turned off or on at different times or in the wrong sequence. For example, the power for a receiver device may be disconnected while leaving the core power supply connected. Due to the nature of the devices, the internal circuit nodes can drift high. In normal operation, internal logic may require two receivers to be in opposite states. Due to the powered-off state of selected receivers, a state may occur which is outside of the design boundaries and result in permanent damage to the chip.
Accordingly, a need has developed in the art for a receiver device that allows the output stage of the receiver device to go to a safe state (e.g., a voltage low) whenever its voltage is disconnected, independent of any of its normal control circuits.
The present invention provides a receiver device having multiple voltage supplies that allows the output stage of the receiver to go to a safe state whenever its voltage is disconnected or powered-down.
Generally, the present invention provides a receiver device comprising:
a first and second voltage supply;
an input circuit using said second voltage supply for receiving an input;
a power sequence circuit using said first voltage supply, said power sequence circuit coupled to said input circuit and outputting an output corresponding with said input when enabled and placing said output into a safe state when disabled; and
an enable circuit, coupled to said power sequence circuit, for enabling and disabling said power sequence circuit corresponding to a voltage level of said second voltage supply.
In addition, the present invention provides a method of producing a valid output for a receiver device having a first and second voltage supply comprising the steps of:
a) sensing a voltage level of said second voltage supply with an enable circuit;
b) producing an enable signal from said enable circuit at a first voltage level when said second voltage supply is above a predetermined value;
c) producing an enable signal from said enable circuit at a second voltage level when said second voltage supply drops below a predetermined value;
d) enabling a power sequence circuit with said enable signal at said first voltage level;
e) disabling said power sequence circuit with said enable signal at said second voltage level;
f) outputting an output of said receiver device corresponding with an input of said receiver device when said power sequence circuit is enabled; and
g) placing said output of said receiver device at a safe voltage level when said power sequence circuit is disabled.
The present invention also provides a system having at least one receiver device comprising:
a first and second voltage supply for each of at least one receiver device;
an input circuit for each receiver device using said second voltage supply for receiving an input;
at least one power sequence circuit for each receiver device using said first voltage supply, each of said power sequence circuit coupled to said input circuit and outputting an output corresponding with said input when enabled and placing said output in a safe voltage level when disabled; and
an enable circuit, coupled to at least one of said power sequence circuit, for enabling and disabling said power sequence circuit corresponding to a voltage level of said second voltage supply.